Nucleases, including zinc finger nucleases and homing endonucleases such as SceI, that are engineered to specifically bind to target sites have been shown to be useful in genome engineering. For example, zinc finger nucleases (ZFNs) are proteins comprising engineered site-specific zinc fingers fused to a nuclease domain. Such ZFNs have been successfully used for genome modification in a variety of different species. See, for example, United States Patent Publications 20030232410; 20050208489; 20050026157; 20050064474; 20060188987; 20060063231; and International Publication WO 07/014275, the disclosures of which are incorporated by reference in their entireties for all purposes. These ZFNs can be used to create a double-strand break (DSB) in a target nucleotide sequence, which increases the frequency of homologous recombination at the targeted locus more than 1000-fold. In addition, the inaccurate repair of a site-specific DSB by non-homologous end joining (NHEJ) can also result in gene disruption. Creation of two such DSBs results in deletion of arbitrarily large regions.
The programmed death receptor (PD1, also known as PDCD1) has been shown to be involved in regulating the balance between T cell activation and T cell tolerance in response to chronic antigens. During HIV1 infection, expression of PD1 has been found to be increased in CD4+ T cells. It is thought that PD1 up-regulation is somehow tied to T cell exhaustion (defined as a progressive loss of key effector functions) when T cell dysfunction is observed in the presence of chronic antigen exposure as is the case in HIV infection. PD1 up-regulation may also be associated with increased apoptosis in these same sets of cells during chronic viral infection (see Petrovas et al, (2009) J Immunol. 183(2):1120-32). PD1 may also play a role in tumor-specific escape from immune surveillance. It has been demonstrated that PD1 is highly expressed in tumor-specific cytotoxic T lymphocytes (CTLs) in both chronic myelogenous leukemia (CML) and acute myelogenous leukemia (AML). PD1 is also up-regulated in melanoma infiltrating T lymphocytes (TILs) (see Dotti (2009) Blood 114 (8): 1457-58). Tumors have been found to express the PD1 ligand (PDL) which, when combined with the up-regulation of PD1 in CTLs, may be a contributory factor in the loss in T cell functionality and the inability of CTLs to mediate an effective anti-tumor response. Researchers have shown that in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), administration of anti-PD1 antibodies blocked PD1-PDL interaction and was able to restore some T cell functionality (proliferation and cytokine secretion), and lead to a decrease in viral load (Barber et al (2006) Nature 439(9): 682-687). Disregulation of PD1 may also play a role in autoimmune disease. SNPs of PD1 (in particular PD 1.3) have also been associated with increased risk for systemic lupus erythematosus (SLE). It has been shown that SLE patients have a higher frequency of the PD 1.3 PD1 allele, and that these patients show reduced PD1 expression on their activated CD4+ T cells (see Bertsias et al, (2009) Arthritis Rheum. 60(1):207-18).
Thus, there remains a need for additional PD1-targeted modulators, for example PD1-targeted nucleases or transcription factors that can be used in research and therapeutic applications.